The present invention relates generally to remote fiber test systems and more specifically to a remote fiber test system having at least one mechanical optical switch for rotatably coupling an optical fiber of a first optical fiber array coupled to a measurement test instrument with a second optical fiber of an opposing optical fiber array coupled to an optical fiber link.
There are generally two types of optical switches in use today: electronic optical switches and mechanical optical switches. Electronic optical switches may be characterized as having no moving parts and perform the switching function, for example, by acousto-optically or electro-optically diverting the light passing through the switch.
Mechanical optical switches, on the other hand, physically move optical fiber elements to produce the switching function. Generally the physical movement of the optical fibers in mechanical optical switches is either transversal or rotational. One family of mechanical optical switches uses focusing elements, such as lenses or the like, to focus the light from one fiber to another. The use of such elements increases the translational tolerances of the switch but substantially decreases its angular tolerances and increases its cost. The other family of mechanical optical switches directly couple the light from one optical fiber to the other. The optical fibers are positioned in opposing relationship with the end faces of the optical fibers in substantially abutting relationship with each other. While this design eliminates the focusing elements and increases the angular tolerances, it substantially decreases the translational tolerance of the switch.
U.S. Pat. No. 4,401,365 describes a rotary-type optical switch in which a pair of opposing optical transmission path mounting members are disposed on the same rotational axis. One mounting member may be fixedly secured in a casing while the other member rotates on a central shaft. Alternately, the shaft may be fixed with one of the mounting members rotating about the shaft. The shaft or the mounting member is directly connected to a motor so that one mounting member is rotatable with respect to the other as the shaft or mounting member is rotated by the motor. The mounting members have respective plane surfaces which are closely opposite each other. Optical fibers are secured in each mounting member such that the end faces of the optical fibers in each mounting member are concentric about the rotational axis of the mounting member and lie on respective phantom circles having the same radii.
U.S. Pat. No. 5,037,176 describes another rotary-type optical switch that includes first and second identical arrays of optical fibers held in axial alignment and relatively rotatable about an axis of rotation to effect optical coupling and decoupling of fibers in the opposing arrays. The optical switch has cylindrical switch bodies that receive the first and second identical arrays of optical fibers. The switch bodies are maintained in coaxial alignment by means of a split sleeve coupler. A tube surrounds the sleeve containing the fiber arrays and O-rings may be disposed between the sleeve and the tube to permit an index matching fluid to be retained within the switch to prevent back reflections. The optical switch described in the '176 patent is incorporated into an optical switch assembly described in U.S. Pat. No. 5,031,994.
A critical factor in mechanical fiber optical switches (MFOS) is the precise alignment of the opposing optical fibers in the switch. Currently, this requires the components of the switch to be made to very precise tolerances along with exacting manufacturing processes. As will be described below, current MFOS fall short in cycle-to-cycle repeatability, long-term repeatability and absolute alignment of the opposing optical fibers.
Mechanical fiber optic switches have unique bearing requirements that are not found in other types of applications. These special requirements need to be examined to understand why current MFOS do not provide the optimum alignment between switching fibers. The alignment tolerances for light coupling between single-mode optical fibers is well known and need not be discussed in detail here. Assuming no longitudinal or tilt misalignment and the input and output fibers are identical, the fractional coupling transmission for optical fibers with lateral misalignment is ##EQU1## where x is the lateral offset and w is the 1/e.sup.2 radius of the irradiance pattern of the fundamental mode of the optical fiber. The derivative of equation [1] is taken to obtain the change in loss for a given change in coupling efficiency. ##EQU2## Equation [2] can be rearranged to solve for .DELTA.x as a function of the lateral offset, radius of the fundamental fiber mode, and the change in loss. The result is ##EQU3##
Using the above equations and assuming a transmission efficiency of the switch must be repeatable within 0.01 dB on a cycle-to-cycle basis with a nominal transmission loss of less than 0.50 dB, maximum alignment tolerance values can be calculated for cycle-to-cycle repeatability, long-term repeatability, and absolute alignment. Since the 1/e.sup.2 radius of the fundamental mode in standard single-mode fiber is roughly 5.0 microns, the nominal loss of 0.50 dB corresponds to a lateral misalignment of approximately 1.7 microns (according to equation [1]). According to equation [3], if the transmission changes less than 0.01 dB on a cycle-to-cycle basis, the misalignment of 1.7 microns must be repeated to within 0.015 microns, or 15 nanometers. The numerical tolerance are calculated for an optical fiber having a mode field diameter of 5.0 microns. Other optical fiber may, for example, have mode filed diameters, such as 5.1 or 5.6 microns. Different mode field diameters will change the numerical tolerances slightly but not substantially.
The 0.015 micron requirement is for cycle-to-cycle repeatability only. There is also a long term repeatability requirement where the transmission efficiency must not change by more than 0.10 dB over about 100,000 cycles. Applying the same analysis using equations [1] and [3], the position accuracy of the opposing fibers in the switch must repeat to within 0.15 microns on a long-term basis or about 1/4th of a wavelength of visible light.
Referring now to FIG. 1A, there is shown an end view of a cylindrical shaft 10 inside a split sleeve 12. In an ideal world, the shaft 10 is perfectly round and has exactly the same outside diameter as the equally perfectly round inside diameter of the split sleeve 12 with the shaft 10 touching the split sleeve 12 along its entire circumference. A bore 14 formed in the shaft 10 for holding the optical fibers is perfectly round and concentric with the shaft 10 and split sleeve 12. FIGS. 1B and 1C illustrate on an exaggerated scale the type of shaft 10, split sleeve 12, and bore 14 that can be expected in the real, imperfect world. None of the elements 10, 12, or 14 will be perfectly round. Instead, shaft 10 and split sleeve 12 will approximate a cylindrical surface, with local regions where the radius is slightly too large, or too small. This is shown in the figures as an ellipse. As can be seen from the figures, the points of contact between the split sleeve 12 and the shaft 10 will change as one or the other rotates, or if any slight lateral torque, as shown by dashed ellipse 16, is applied to the shaft 10, so that fibers (not shown) in the shaft will not trace out concentric circles. Notice also that, at the point of contact, the surface of the split sleeve 12 is parallel to the surface of the shaft 10. The only force preventing the shaft 10 from slipping in the split sleeve 12 is the frictional force between the two surfaces. The frictional force is incapable of reliably providing the kind of cycle-to-cycle or long-term repeatability that is needed. Furthermore, there is the paradox of lubrication. In order to extend the life of the bearing surfaces it is desirable to lubricate them, but lubrication reduces the frictional forces between the two surfaces, resulting in more wobble.
FIGS. 1B and 1C illustrate an additional problem. The fibers align themselves to the shaft 10 via the bore 14 drilled along the axis of the shaft 10, and this bore 14 has its own set of tolerances. Specifically, the bore 14 will be slightly out of concentricity with the outside surface of the shaft 10, and like the outside surface of the shaft 10, it will be slightly out-of-round.
There are multiple dimensional tolerances that must be tightly specified if the input and output fibers of the switch are to rotate on identical circles that are precisely concentric. The design parameters that must be firmly controlled are:
Roundness of the input shaft outside diameter.
Roundness of the output shaft outside diameter.
Roundness of the input shaft inside diameter.
Roundness of the output shaft inside diameter.
Concentricity of the input shaft inside and outside diameters.
Concentricity of the output shaft inside and outside diameters.
Outside diameter of the input shaft.
Inside diameter of the input shaft.
Outside diameter of the output shaft.
Inside diameter of the output shaft.
Inside diameter of the split sleeve.
Roundness of the split sleeve inside diameter.
Diameters of the input and output fibers.
Concentricity of the input and output fibers.
To maintain an insertion loss of less than 0.50 dB, all of these tolerances must add up to less than about 0.17 microns of misalignment. This is an extremely difficult task, and to accomplish it the individual components (input fibers, output fibers, input shaft, output shaft, and split sleeve) must have several dimensional tolerances that are sub-micron. This is certainly not conducive for minimizing the costs of individual components, and is daunting in terms of manufacturability.
Another issue in mechanical fiber optic switch design is switch repeatability. Referring to FIG. 2, there is shown a side view of the shaft 10 and sleeve 12 of FIGS. 1B and 1C with the sleeve 12 being sectioned. The split sleeve 12 works with shaft 10 that is slightly larger than the inside diameter of the unexpanded sleeve 12. Because the sleeve 12 is split, it can expand slightly to allow the shaft 10 (a ferrule containing the optical fibers) to fit inside with no diametrical clearance. Diametrical clearance is unsatisfactory because it results in slop within the bearing, and there is needed less than 0.015 microns of misalignment non-repeatability between the opposing fibers to meet the cycle-to-cycle repeatability specifications.
As has been discussed with FIGS. 1B and 1C, the out of roundness on the part of the shaft 10 and the sleeve 12 will cause the fiber to move on curves that are not circles. However, barring wear in the bearing, out-of-roundness should not result in slop or lack of repeatability. Out-of-roundness will affect the total coupling efficiency, but not the repeatability. FIG. 2 shows the shaft 10 having an interference fit with the sleeve 12. However, a second shaft 18 will most likely have a slightly different diameter owing to the inevitable tolerances in manufacturing. If the second shaft 18 has a larger diameter than the first shaft 10, then it will expand the split sleeve 12 a little bit, resulting in an interference fit for the second shaft 18 but not the first shaft 10. Now the first shaft 10 can slop in the split sleeve 12. If the second shaft 18 has a smaller diameter than the first, then it will wobble. No matter what happens one of the two shafts 10 or 18 will wobble within the split sleeve 12. To meet the cycle-to-cycle repeatability requirement this wobble must be less that 0.015 microns, so the diameter of the two shafts 10 and 18 must be equal to about 0.008 microns. This specification would require extremely expensive parts. However, for all practical purposes, meeting such a specification would be impossible to do.
What is needed is an inexpensive mechanical fiber optic switch that meets the cycle-to-cycle repeatability, long-term repeatability, and absolute misalignment specifications. Such a switch should use loosely toleranced commercially available off-the-shelf components and be easy to assemble without requiring fine alignment of the switch components and fibers. In addition, the switch should have a fiber mounting system that has minimum bearing wear and is insensitive to dimensional differences of the components. Further, the switch should have good stability over temperature.